The content clearly matters, and efficacy is tracked (this year it was poor because the eventual pandemic flu strain was a H3N2 virus which mutate rapidly)[0]. This was despite WHO updating the recommendations at the last hour in April/May 2025.

But critically this isn't as important as people think. The primary goal of the flu vaccination is of course to temper spread of the main viruses that season. But it's also to build people's immune library of exposure to flu viruses.

Recall that the 1918 "Spanish" flu was so terrible not because it was intrinsically a worse virus but that it was one which many younger generations had not been previously exposed.

COVID has meant that many younger generations again has a much smaller library of past exposure.

[0] https://www.cidrap.umn.edu/influenza-vaccines/estimated-effe...